Would you have believed that Matthew Hicks (left) was actually Prince Harry?Photo: Startraksphoto; Getty Images

How do you convince a dozen 20-something American women they’re competing for the heart of Prince Harry? Put them up in an English estate, helicopter in a guy that looks like the fourth in line to the throne — and let them jump to their own conclusions.

That’s the premise of the dating show “I Wanna Marry ‘Harry,’ ” which premiered Tuesday on Fox: The ladies are told only that their heartthrob is an eligible bachelor, referred to by his butler as “Sir.” Asked by one woman what they should call him, he suggests, “Whatever you like.”

But “Harry” is actually Matt Hicks, a 23-year-old average English bloke who happens to be the spitting image of the party-loving prince.

So who in the world would believe Prince Harry would actually look for love on American television? The funny thing is, not even the competing girls — at least not at first. But as the secrecy went on for more than a month (and as they were cut off from TV and the Internet), even the most skeptical doubted their instincts. And now they’re defending themselves.

Hicks dances with Karina at a masquerade ball during the season premiere of “I Wanna Marry ‘Harry.'”Photo: Chris Raphael/FOX

“When you’re trapped in a world where there’s no contact with the outside world . . . you’re going to eventually believe it,” says contestant Jacqueline Conroy, 25, a New York City graduate student.

Producers did all they could to reinforce the charade, staffing “Harry” with a security detail and being secretive about his identity.

“It was reverse psychology, I guess,” Hicks says. “It’s a much more powerful method of persuasion to let people come to their own decisions and conclusions than to say outright this guy is Prince Harry on a dating show.”

Hicks received a crash course in etiquette. He also spent a week learning the details of Prince Harry’s life — everything from his military career to past scandals (like being photographed naked in Las Vegas). His familiar looks, however, came naturally.

“I had [the resemblance] mentioned here and there as I was growing up, but it was when I got to university when it really kicked off,” Hicks tells The Post.

Jacqueline (center) says she was just hoping for a good story, and Kimberly (left) admits she thought it might have been a hoax. Kelley (right) says she was disappointed to learn he wasn’t the real prince.Photo: Daniel Smith/FOX

After college, he posted some pictures on a lookalikes website at the urging of a friend, and started getting small jobs to impersonate His Royal Highness at events. A natural blond, Hicks dyes his hair ginger when making “Harry” appearances, but says he shares the prince’s style.

Hicks talks with the contestants during the season premiere.Photo: Chris Raphael/FOX

Before the show, he never used the resemblance to land a girlfriend, though it did help him get the ladies’ attention.

“Automatically people would just notice, and it was an icebreaker more than anything,” Hicks says. “In the UK, I don’t think people actually thought I was him. They found it quite entertaining.”

He told TV Guide he did the show mainly to be treated like a prince and meet some American girls — “If I find a romantic interest along the way, all the better.”

Some of the women readily admit they weren’t primarily looking for love, either. Conroy was cast in the show with her friend Kimberly Birch, 24, an actress from Malverne, LI. Both had serious boyfriends in the past and signed up to have a good story to tell — which allowed them to be good sports when they learned the truth.

“When they actually told us that he wasn’t Prince Harry, at that point I had already thought that,” Birch says.

The REAL Prince Harry speaks with locals during a trip to Estonia.Photo: Getty Images

Kelley Andrews, a 24-year-old media researcher in New York City, went into the show with a crush on the real Prince Harry. She wasn’t finding the Southern-gentleman type she desired from her Mobile, Ala., upbringing — and admits she was charmed by Hicks.

Without revealing whether she bought into the hoax, she says she wasn’t disappointed after learning his true identity.

“People ask questions like, ‘How couldn’t you know it’s not him?’ I don’t mind all the things like ‘royal hoax,’ ‘these women are silly Americans,’ ” she says. “I had a great time.”